arrow

ARROW-8922: [C++] Add illustrative "ascii_upper" and "ascii_length" scalar string functions valid for Array and Scalar inputs

There's some new code generation machinery here (that will be worth ongoing iteration) but the relevant implementation / "developer UX" is what's in string_scalar_ascii.cc, take a look.

Note: the implementation of `ascii_upper` is far from optimal. `std::toupper` does more than convert ASCII to uppercase and so it would likely be faster to replace it with a bespoke implementation that only deals with the ASCII alphabetic character space

```
In [1]: import pyarrow as pa; import pyarrow.compute as pc

In [2]: arr = pa.array(['aaa', 'bbbbbb', None, ''])

In [3]: pc.ascii_upper(arr)
Out[3]:
<pyarrow.lib.StringArray object at 0x7f7044003e50>
[
  "AAA",
  "BBBBBB",
  null,
  ""
]

In [4]: pc.ascii_length(arr)
Out[4]:
<pyarrow.lib.Int32Array object at 0x7f7044003910>
[
  3,
  6,
  null,
  0
]
```

int64 offsets are respected with LargeString

```
In [5]: arr = pa.array(['aaa', 'bbbbbb', None, ''], type='large_utf8')

In [6]: pc.ascii_length(arr)
Out[6]:
<pyarrow.lib.Int64Array object at 0x7f703c74cbb0>
[
  3,
  6,
  null,
  0
]
```

Closes #7278 from wesm/ARROW-8922

Authored-by: Wes McKinney <wesm+git@apache.org>
Signed-off-by: Wes McKinney <wesm+git@apache.org>